Measuring Air Pressure on front bumper??

JimIsbell · 10-17-2008, 01:15 AM

I am interested in putting a pitot tube on the front of the car to see what the air pressure is at different speeds. Then I want to make the same measurements while drafting on a truck at different distances back. The reason I am interested in this is that I seem to find at least TWO sweet spots for drafting and the second one is nicely back and safe. I think there is a standing wave pattern behind a truck and if I can map it it will help to utilize drafting as a SAFE technique.

What I need is a reasonable way to measure it.

First, does anyone have any idea what the air pressure at the front of the car might be at , say 55 mph?

Second, does anyone have an old aircraft air speed indicator or altimeter that they might be willing to part with cheap?

Third, does anyone have any other ideas for a reasonable measurement device that wouldn't brake the bank?

**IamIan** · 10-17-2008, 04:54 AM

good luck... sounds like a good project... I have no idea what you would use though... aside from the durability issues of mounting anything there... I suspect that there will be a fair amount of turbulence in the air, which would lead to very chaotic readings... even after you figure what to use and how to mount it, interpreting the data may not be straight forward.

boogetyboogety · 10-17-2008, 07:54 AM

Hmmmmm... intriguing idea... and you just reminded me of something.

Years and years ago, my brother decided to run for public office (got as far as the primaries!).

On one of his many campaign events, we were caravaning... I was following a big cube van that held his PA system and other large equipment down a highway, driving our late, lamented Subaru GL wagon. Amongst other embarrassing political and patriotic decorations all over our vehicles, I had mounted two 12"X18" American flags (on little tube brackets I concocted) right at the front of the hood, on either side of the faired-in headlights. To make a long story short, My Beloved happened to notice that at a certain spot behind the van (and not really close, either), the flags fluttered differently than when the distance increased or decreased, and I remember making some silly comment about "that's why racers draft other cars..."

I wonder if something as primitive as that could give us some rough idea of where the "sweet spot(s)" exist...

JimIsbell · 10-17-2008, 04:20 PM

My pitot will be forward for the front of the car so car induced turbulence wont be a problem in the first set of measurements. In the second set, drafting, there will be turbulence from the truck but I think that most measuring instruments such as air speed indicators or altimeters will have enough hysteresis that the noise will be filtered out.

Willie Williford · 10-17-2008, 11:38 PM

For my "drafting" I use sound as a guide.
I drive with the drivers window down about 2 inches. There is the sound of the wind rushing by. When I reach the "sweet spot" there is a definate change in the pitch of the wind .

Try it !

Willie

JimIsbell · 10-18-2008, 10:36 AM

Yes, that works for me too, but I want to verify the standing wave theory and this is the only way to get definitive data.

Aaron Cake · 10-18-2008, 10:38 AM

You can make or purchase a cheap magnehelic gauge. A homemade version would be a U shaped tube full of water, with one end of the tube reaching the front bumper. Probably very hard to read on the road though.

Commercial versions are dial indicators, fairly cheap as well.

JimIsbell · 10-18-2008, 11:31 AM

I thought of the water in a tube method, but I didn't think about it being hard to read. I am glad you brought that up...

I am now thinking about the carb synchronizer I have, or making something with a vane, pot, spring and meter. I could vary the size of the vane as needed to calibrate it. Since its only temporary I don't care about looks. And I don't need a real measurement, only a relative measurement.

retepsnikrep · 10-18-2008, 01:23 PM

Could you use a stethoscope and listen to the sounds as you drive?

I use one to listen to engine noises and such like.

Or going back to WWII

You can move the reciever end about and experiment with trying to acheive lowest noise zone.

JimIsbell · 10-18-2008, 02:22 PM

Since what holds the car back is pressure, not noise, I think a pressure reading would be more meaningful....besides I don't want to drill a hole in the roof to mount that thing. It looks heavy...maybe 1000 pounds..?..